leetcode 720 - Longest Word in Dictionary

class Solution {
public:
    string longestWord(vector<string>& words) {
        string best;  //存储当前最优解
        //unordered_set<string> dict(words.begin(), words.end());
        for(const string& word: words){
            //pruning
            if(word.length()<best.length() || (word.length()==best.length()&& word>best))
                continue;
            string prefix;
            bool valid = true;
            for(int i=0; i<word.length()-1 && valid; ++i){
                prefix += word[i];
                if(find(words.begin(), words.end(), prefix)==words.end())
                //if(!dict.count(prefix))
                    valid = false;
            }
            if(valid) best = word;
        }
        return best;
    }
};